Lubricating oils for use in crankcase engine oils contain components that are used to improve the viscometric performance of the engine oil, i.e., to provide multigrade oils such as SAE 5W-30, 10W-30 and 10W-40. These viscosity performance enhancers, commonly referred to as viscosity index (VI) improvers include olefin copolymers, polymethacrylates, styrene/hydrogenated diene block and star copolymers and hydrogenated isoprene star polymers.
Olefin copolymers (or OCP) used as viscosity index improvers conventionally comprise copolymers of ethylene, propylene and, optionally, a diene. Small polymeric side chains do not exert a substantial viscosity modifying effect in oil. Polymerized propylene has one methyl branch for every two backbone carbon atoms. Ethylene polymer is substantially straight chained. Therefore, at a constant amount of polymer in oil (treat rate), an OCP having a higher ethylene content will display an increased high temperature thickening effect (thickening efficiency, or TE). However, polymer chains having long ethylene sequences have a more crystalline polymer structure. Crystalline polymers are less oil soluble than amorphous polymers and as a result, the OCP can drop out of the oil, causing the plugging of engine filters (and the loss of the viscosity modifying effect). Further, OCPs having substantial crystalline segments have been found, again primarily at low temperatures, to interact with waxes in the oil, and other OCP chains, which results in uncontrollable increases in low temperature viscosity and, in extreme cases, the gelling of the lubricating oil. These problems have been found to manifest in Ziegler Natta polymerized OCPs containing greater than about 60 wt. % ethylene (hereinafter referred to as “high ethylene content OCP(s)”).
Due to their molecular architecture, star polymers are known to provide improved shear stability compared to OCPs. VI improvers that are star polymers made by hydrogenation of anionically polymerized isoprene are commercially available. Anionic polymerization results in a relatively low molecular weight distribution (Mw/Mn). Hydrogenation results in alternating ethylene/propylene units having a composition comparable to a polymer derived from 40 wt. % ethylene and 60 wt. % propylene. These VI improvers provide excellent shear stability, good solubility and excellent cold temperature properties. However, improvements in the balance between the thickening efficiency and shear stability of star polymers remain possible.
U.S. Pat. No. 4,116,917 exemplifies certain star polymers comprising hydrogenated poly(butadiene/isoprene) tapered arms containing about 44.3 wt. % polymer derived from butadiene. Since butadiene initially reacts faster than isoprene when anionic polymerization is initiated with secondary butyllithium (the process described in the patent), a polybutadiene block is first formed. As the butadiene concentration is lowered through polymerization, isoprene begins to add to the living polymer so that, when the polymerization reaction is complete, the chain is made up of a polybutadiene block, a tapered segment containing both butadiene and isoprene addition product, and a polyisoprene block resulting in a living tapered polymer chains that, when coupled with divinylbenzene, produce a star polymer having a polybutadiene block positioned distal from the divinylbenzene-coupled core. The long ethylene sequences of these external hydrogenated polybutadiene blocks were found to cause the same low temperature performance problems associated with high ethylene content OCPs.
To provide an improvement in thickening efficiency, while maintaining low temperature performance, U.S. Pat. No. 5,460,739 suggests star polymers comprising triblock copolymer arms of hydrogenated polyisoprene/polybutadiene/polyisoprene. The hydrogenated polybutadiene block provides an increased ethylene content, which improves thickening efficiency. The patent suggests that, by placing the hydrogenated polybutadiene block more proximal to the nucleus, the adverse effect on low temperature properties could be minimized. Such polymers were found to provide improved low temperature properties relative to the tapered arm polymers of U.S. Pat. No. 4,116,917. However, when such polymers were provided with a hydrogenated polybutadiene block of a size sufficient to provide a credit in thickening efficiency, a debit in low temperature performance remained relative to the pure polyisoprene polymers.
Therefore, there remains a need for improved VI improvers that are capable of providing a TE comparable to that of high ethylene content OCP, without the associated reduction in solubility and cold temperature performance.